A typical EUV light source device that generates extreme ultraviolet light in a conventional way (shown as a simple schematic diagram in FIG. 15) includes an EUV chamber that is kept in a vacuum, and a device for droplet generation that ejects droplets of a target material which radiates EUV when turned into plasma. The target material is turned into plasma through irradiation of a pulsed driver laser, whereupon the EUV light radiated by the plasma is focused to a focal point by way of a collector mirror. The focused EUV light propagates next into a semiconductor exposure device using EUV light, and is eventually guided onto a semiconductor wafer.
The position of the focal point is a predetermined position in the semiconductor exposure device. Accordingly, the EUV generation point must be fixed at a predetermined position within the EUV light source. That is, the driver laser and the target material droplets must interact (i.e. the target material droplets must be irradiated by the laser beam) at the EUV generation point at all times.
Examples of the target material include, for instance, liquid metal from Sn, Li, or the like, melted through heating at a temperature at or above the melting point (Sn: 232° C., Li: 180° C.), or a dispersion of micro-particles of Sn, SnO2, or the like, in a solvent such as water or alcohol.
Generation of target material droplets in the droplet generation device is beset by the following problems. The nozzle that forms the droplets may become clogged on account of changes in the surface condition of the nozzle, or through intrusion of impurity particles into a flow channel. When using liquid metal in the target material, moreover, a target material at high temperature flows through the nozzle, which may give rise to thermal deformation of the nozzle. As a result, the ejection direction of target material droplets from the droplet generation device becomes unstable, which may preclude supplying target material droplets stably to the point of interaction with the laser beam, i.e. the EUV generation point. EUV light cannot be generated stably when such problems occur.
Methods such as the one disclosed in, for instance, US 2005/0199829 A1 attempt to solve the above problems.
Other proposed methods involve selectively charging some of the small droplets that are continuously jetted out of a droplet generation device and deflecting the charged droplets by way of a parallel electric field, in order to widen the spacing between the small droplets, so that only charged droplets are taken out of the droplet stream and are supplied to the EUV generation point. (Japanese Patent Application Laid-open No. 2007-200615 and US 2008/0048133 AI).
In US 2005/0199829 A1, the position of target material droplets is monitored by a plurality of position sensors (CCD cameras, or the like). When the droplet position is offset from the EUV generation point, the droplet generation device is displaced, on the basis of information regarding that offset, in such a manner that the droplets pass through the original EUV generation point. Displacement of the droplet generation device is accomplished, for instance, by way of a droplet generation device position control device employing a stepping motor, or the like, and mounted on the droplet generation device. FIG. 16 illustrates schematically an instance of droplet position correction based on the abovementioned prior art method. The position sensor of target material droplets and the position control mechanism of the droplet generation device are omitted from FIG. 16.
Changes in the ejection direction of the droplets can be corrected by controlling the motion of the droplet generation device itself, so long as the change is comparatively slow, for instance, a drift-like direction change. However, the control of the droplet generation device cannot cope with instantaneous direction changes that occur faster than the time interval at which the motion direction of the droplet generation device is controlled (for instance, 0.03 s). Therefore, in the above method, as well, there exists a time window during which droplets do not pass through the laser irradiation point, and hence the problem of unstable EUV output remains unresolved.
A further drawback is that the method requires, for instance, equipment for measuring droplet position, and a control mechanism, a controller, or the like, for controlling the motion of the target generator, as described above, all of which results in an overall larger EUV device.
In case of changes in the ejection direction of droplets from the droplet generation device, droplets deflected by way of a deflecting electrode may fail to pass through the EUV generation point, as illustrated in FIG. 17, which results in the same problem of unstable EUV output, even when using the droplet selection techniques disclosed in Japanese Patent Application Laid-open No. 2007-200615 and US 2008/0048133 A1 as offset correction schemes that rely on displacing the droplet generation device, as described above.